Acousto-optic (AO) technology utilizes diffraction effects caused by acoustic strain waves in a block of suitable material (referred to as the AO interaction medium) to effect control over electromagnetic waves. Typically a strain wave is launched into a suitable AO material capable of supporting both the strain wave and the optical wave in the same region. The acoustic strain wave imparts a perturbation to the index of refraction in the AO medium relating to the propagation of the electromagnetic wave. By controlling the amplitude of the acoustic strain wave, control over the electromagnetic wave can be achieved.
Heat generation in AO devices is known arise from three main sources, optical absorption, acoustic absorption and Ohmic heating at the transducer. Minimizing these sources of heat in order to maintain device performance and stability is generally an AO device design goal.
Both optical absorption and acoustic absorption generate heat within the AO medium and must be thermally conducted out of the material. This conduction process is limited by the thermal conductivity of the AO medium and the thermal conductivity of the surrounding heat-sink material. Once a specific AO medium and acoustic propagation mode has been chosen, the optical absorption and acoustic absorption are fixed. The third heat source, Ohmic heating, is generally controlled by good transducer design.
In typical AO Q-switching devices, the radio frequency (RF) power applied to the acoustical transducer required to produce enough diffraction to stop the laser oscillating is generally at least on the order of several tens of Watts. In such cases, air cooling is no longer viable and water cooling generally becomes necessary.
Water cooling is both for the removal of heat from acoustic absorption and Ohmic heating. This absorption of the acoustic power after the passage of the acoustic wave through the AO medium is vital to the correct operation of the AO Q-switch. If the acoustic power is not all absorbed, then some will be reflected, where it will encounter the laser beam a second time, generally after a delay of several micro-seconds, and thus can cause diffraction a second time. This effect is unwanted, and generally leads to unreliable effects (“after-pulsing”) and lack of control of the laser. Acoustic absorbers are known in the art to reduce acoustic reflection to some degree.
Acoustic energy can be absorbed in the AO medium by virtue of its intrinsic acoustic absorption. This results in heat generation within the AO medium which should be subsequently be conducted out of the AO medium in order to limit any temperature rise to an acceptable level. As noted above, in typical AO devices, the acoustic power in the devices is typically tens of Watts. If all of this power is converted to heat within the AO substrate (and then removed by thermal conduction) a significant temperature gradient will result across the AO medium. Significant temperature gradients are known to degrade properties of the AO device. It is thus generally better to instead try and extract the acoustic energy from the AO material, such as into an acoustic dump/heat sink which is cooled by forced air/conduction/water cooling.
Absorption of the acoustic energy directly in the heat sink material can be advantageous for thermal management of the overall structure as the thermal conductivity of the heat-sink material can be chosen to be significantly higher than that of the AO medium. A number of materials are available for this type of acousto-optic interaction. For high power lasers operating in the region of 1 μm, the main choices are generally crystal quartz and fused silica. Fused silica is used as an example below to explain the main design steps and considerations.
For stable performance of AO devices, and in particular some AO Q-switches, the heat generated during operation must be extracted and the device temperature rise limited to acceptable levels. A very common method of cooling is to attach metal cooling plates to the AO medium in positions which will not interfere with the straight-through passage of the optical beam from its input face to output face of the AO crystal. The plates are generally cooled by flowing water through enclosed channels formed in the cooling plates. A pump generally maintains the flow of water. The mechanical, thermal and acoustic properties of the chosen metal plate material are important to achieve optimum device performance. The particular material is generally chosen based on a combination of these properties. In particular, the key cooling-related parameters of the material are the specific acoustic impedance (ZO) and the thermal conductivity (K).
As known in the art, if two different materials are joined together at a plane interface, and an acoustic wave is propagating in one of them such that it encounters the interface region, the amount of acoustic power reflected at the interface is known to depend on the acoustic impedance of the two layers, ZO,1, ZO,2. If ZO,1=ZO,2, then in general there will be no reflection at this interface, otherwise the proportion of acoustic power reflected will depend on the ratio ZO,1/ZO,2. Thus, if an AO interaction medium of characteristic impedance ZO,1 is joined to a heat-sink/acoustic dump material having a characteristic impedance ZO,2 there will be a possibility of complete absorption of the acoustic energy directly in the heat sink only if the two acoustic impedances are equal. It is noted that for a particular material, ZO=ρV, where ρ is the density of the material and V is its acoustic velocity. Thus, even if the AO interaction medium and heat sink are different materials (e.g. silica and aluminum, respectively), as long as the product of ρ and V in each is the nearly same, the heat sink will reflect a minimal amount of acoustic power back into the AO medium, as required.
Considering mechanical, thermal and acoustic properties of the cooling plate material often results in the selection of aluminum or a similar material or alloy (e.g. aluminum alloy). A common problem with aluminum comprising plates is the onset of corrosion (oxidation) in the cooling channels caused by the interaction of the aluminum with the water and with other metals that may be present within the water system. It is difficult to stop this corrosion by altering the pH of the cooling fluid, because aluminum has the unusual property of being vulnerable to corrosion in neutral, as well as both low pH and high pH aqueous solutions.
The major effects of the corrosion include leaks at joints, injection of corrosion products into the fluid flow, blockage of cooling channels, and reduced operating lifetime. All of these factors can result in deterioration in system performance and can result in the failure of the Q-switch (due to insufficient cooling), or the failure of other components utilizing the same coolant circuit.
A number of “solutions” have been disclosed that attempt to minimize corrosion-related problems. All known solutions have at least one significant shortcoming. Several common “solutions” are described below.
It is common practice for AO devices, including AO-based Q-switches, to metal plate the cooling blocks with a corrosion resistant material with the intention of creating a barrier layer between the corrosion sensitive cooling block material (e.g. aluminum alloy) and the surrounding environment. For example an anodized or an electroless plated nickel finish can be applied to aluminum and its alloys. Although the intent is to protect the both the interior and exterior surfaces of the cooling channels from oxidation associated with coolant flow, due to the nature of plating processes, although the exterior surfaces are generally uniformly plated, the interior channel surfaces are not fully covered with plating material due to the difficulty in getting the plating material to extend down the inside of the channels. The inability to properly plate in the interior surfaces of the cooling channels plagues both electrolytic and non-electrolytic plating processes. As a result, metal plating provides limited effectiveness in terms of preventing corrosion. Moreover, coolant flow can cause erosion-corrosion of the barrier material that is present inside the cooling channels. In this process, small particulates in the coolant flow continually impinge on the inner walls of the cooling channels, thus eroding the plating and expose the underlying metal which then corrodes at an even faster rate. Moreover, heat is less efficiently extracted in the plated arrangement because the corrosion resistant plate material which ends up on the outside surfaces (sides of the block) of the cooling block material and thus becomes in physical contact with the optical medium upon assembly has a smaller K as compared to the bulk coolant material (e.g. aluminium).
Materials other than Al or Al alloys that are more resistant to corrosion have been used and a good deal of success has been achieved. However, although such alternate materials have been successful in delivering a Q-switch design more resistant to corrosion, the overall device performance has been limited. The limitation arises because the good mechanical, thermal and acoustic properties are not present simultaneously. Typically, heat is less efficiently extracted because the corrosion resistant plated material has inferior properties (smaller K) as compared to most alternate heat-sink/acoustic dump materials (e.g. aluminum). If less heat can be extracted then it follows that less RF drive power can be applied to the device and the device efficiency is compromised (device efficiency being proportional to the electrical drive signal strength).
Increased coolant flow rate is a possible solution but this option is often not available. In addition increased coolant flow comes at an additional cost, such as larger and more expensive water pumps.
Cooling plates are often made from solid pieces of metal which have flow channels (‘ways’) drilled through them to form continuous channels which carry the coolant from one side of the plate to the other. In drilling the ‘ways’ it is necessary to drill holes deep into the material which are later blocked with plugs near the surface in order to make one or more leak-free continuous cooling channels. A significant problem with coolant plates having ways is leakage of coolant at these plugged holes either due to poor sealing or due to local corrosion.
What is a needed is a corrosion resistant fluid cooled arrangement for AO devices which provides high thermal conductivity and an acoustic impedance close to the value of acoustic impedance for the AO medium. In addition, such an arrangement should be a relatively low-cost arrangement.